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Variations of reservoir water level and seasonal precipitation have resulted in significant movement and destabilization of landslides in the Three Gorges Reservoir (TGR) region of China since reservoir impoundment in 2003. An example is the Baijiabao landslide, a large, actively creeping landslide located in the steep lower valley of the Xiangxi River, about 55 km upstream of the TGR dam in the Yangtze River. Twelve years of monthly monitoring at four GPS stations and routine, monthly field observations show cumulative GPS displacements as large as >1.5 m and widely developed ground cracks. GPS monitoring results show that most movement takes place in rapid steps that coincide with the rainy season and the period of annual reservoir drawdown, with particularly large steps in 2009, 2012 and 2015. This step-like pattern of displacement is also shown by daily data from an automatic monitoring system installed in 2017. The total period of acceleration shown by these daily data was about six weeks long, with rapid movement starting during rapid reservoir drawdown, and terminating when the reservoir began to rise again. In particular, most of the 2018 displacement occurred in only two weeks. Different subzones of the landslide move at different rates and exhibit different features of deformation. The neighborhood rough set theory is used to identify the triggering factors responsible for landslide deformation. The most important triggering factors vary between different sites, data types and the time interval used to define them. The surface deformation and ground crack widening are controlled by the combination of rainfall and variations in the reservoir water levels, whereas the deformation of the sliding zone is most sensitive to the latter. The results show that daily data are needed to capture important, short-term landslide responses. The neighborhood rough set theory for determination of triggering factors is suggested for deformation prediction, stability evaluation, and prevention and control of reservoir landslides in this and other regions. • Spatiotemporal deformation characteristics of Baijiabao landslide over a twelve-year period are analyzed in detail. • Different acceleration periods and threshold hydraulic conditions are observed from daily and monthly monitoring data. • Neighborhood rough set theory is used to identify the triggering factors responsible for landslide deformation. • The most important triggering factors vary between different sites, data types and the time interval used to define them. [ABSTRACT FROM AUTHOR]

Cyclic interactions between rocks and water have general significance and are particularly relevant to engineering failures and geohazards. A comprehensive investigation of the multi-scale deterioration effects and mechanisms of cyclic wetting-drying on rocks can contribute to better engineering applications, but the probabilistic characterization of rock mass properties under cyclic wetting-drying remains a problem due to the difficult-to-test parameters and limited data. Numerous experiments on rock samples of argillaceous siltstone subjected to cyclic wetting-drying of Yangtze River water demonstrate progressive alteration of rock properties on multiple scales, including changes in mineralogy, grain structure, computed tomography, microporosity, and complexity of micropores, with calcite and albite being the most active minerals. The uniaxial compression strength and indirect tensile strength decrease exponentially with the cyclic number, while the Young's modulus increases. Considering the probabilistic characteristics of the geological strength index, the probabilistic mechanical properties of rock mass of studied argillaceous siltstone are then estimated through the generalized Hoek-Brown criterion. The results indicate that the uniaxial compression strength and indirect tensile strength of rock mass both follow lognormal distributions, while the Young's modulus follows a skewed distribution. Long-term strength parameters of rock and rock mass can also be determined as they exhibit exponential trends with the cyclic number. During the cyclic wetting-drying process, the water-rock interactions produce progressive changes in mineral compositions, microstructures, and micropores; with the accumulation of variations of physical properties, defects grow and develop into weak zones, thus the mechanical properties of argillaceous siltstone are significantly weakened. Investigations of the wetting-drying effects on the geological strength index and probabilistic characteristics of rock sample properties are recommended to accurately evaluate rock mass properties affected by cyclic wetting-drying. • Argillaceous siltstone is treated by cyclic wetting-drying of Yangtze River water. • Dissolution of calcite and albite results in the synergistic evolution of rock properties. • Micropores with a diameter greater than 100 μm become predominant. • Probabilistic properties of rock mass strength under cyclic wetting-drying are evaluated. • Long-term strength of the rock mass is evaluated through the exponential fitting. [ABSTRACT FROM AUTHOR]

• A modified Bayesian sequential updating approach is proposed to estimate GSI. • Multi-source information can be integrated in the approach. • GSI can be estimated using limited data based on various rock mass classifications. • Evaluated probabilistic characteristics of the GSI agree with the observed GSI data. • Rational result is achieved by taking model uncertainties as random normal variables. Explicit determination of geotechnical parameters is an important but difficult task in rock engineering. Paradoxically, data are often limited even though data from multiple sources (e.g., different testing procedures, testing positions, and estimation models) are commonly available, and the integration of multi-source information for the determination of geotechnical parameters in a probabilistic context remains a great challenge. We have modified an existing Bayesian sequential updating approach and used it for the first time to estimate the geological strength index (GSI) of rock masses by integrating multi-source information in a way that considers prior information, multiple estimation models and probabilistic properties of model uncertainties. The argillaceous siltstone in Zigui County in the Three Gorges Reservoir region of China was used to quantitatively illustrate this method. Three data sets using rock mass rating (RMR)-, tunneling quality index (Q)-, and rock mass index (RMi)-based estimation models were sequentially incorporated into the Bayesian sequential updating framework, through which the information on the mean value and standard deviation of the GSI can be updated. Then a large number of equivalent GSI samples were generated through Markov chain Monte Carlo (MCMC) simulation for further statistical analysis. The results showed that the proposed Bayesian sequential updating approach can provide reasonable probabilistic estimates of the GSI, which compared well with observed data using the standard GSI chart. The Bayesian sequential updating approach is a promising tool for geotechnical parameter estimation and can combine large amounts of information, thereby effectively depicting the probabilistic characteristics of GSI. [ABSTRACT FROM AUTHOR]

As a consequence of seasonal variations in groundwater level and reservoir water level, natural rocks generally experience the cyclic wetting-drying process, in which the change in microstructure becomes a critical factor leading to the deterioration of the macro-physical and mechanical properties of rocks. However, it remains a great challenge to quantitatively characterize the microstructures of rock, especially during the wetting-drying process. In this study, a systematic experimental investigation involving multiple techniques (mercury injection capillary pressure (MICP), computer tomography (CT), scanning electron microscope (SEM), and pores (particles) and cracks analysis system (PCAS) software package-based image analysis) were conducted to study the microstructure of sandstone subjected to cyclic wetting-drying. The results show that sandstone experiences increasing microporosity during the cyclic wetting-drying process, with progressive changes in micro grains, defects, and micropore structure and morphology. The pore compositions gradually change into a bimodal distribution with dominating small-medium pores (10 nm-10 μm) and large pores (>10 μm). Eventually, connected crack networks and large-scale fractures develop. There is a strong correlation between the microporosity values evaluated by CT and MICP, indicating the capacity of describing the micropore compositions ranging from 0.03 to 0.78 mm. Besides, comparisons of the three techniques (i.e., MICP, CT, and SEM) and evolution mechanisms of sandstone microstructures are further discussed. • Sandstone microstructures under cyclic wetting-drying are studied via multiple techniques. • Pore area, complexity, and microporosity gradually increase, and defects grow. • Micropores have a bimodal distribution with small-medium pores and large pores being dominant. • Micropores show morphological anisotropy in three dimensions. • CT agrees with MICP in describing micropore compositions ranging from 0.02 to 1 mm. [ABSTRACT FROM AUTHOR]

Most soft interlayers tend to be key factors affecting rock slope stability due to their low strengths. A series of ring shear tests is conducted on reconstituted overconsolidated soft interlayers to investigate the influences of remolded water content and normal stress on shear behavior under drained conditions. The soft interlayer raw samples taken from a potential landslide site in China are first remolded into overconsolidated samples with different water contents but the same dry density and further drained under different normal stresses. The peak strength, residual strength, corresponding shear displacement of the soft interlayers and the loss rate of water content are positively correlated with water content and normal stress. Transformation of strain softening to strain hardening occurs for samples under higher normal stresses. Variations in water contents have notable effects on the shear strengths. Meanwhile, displacements, fragmentations and orientations of samples differ, showing strain softening and hardening behaviors of soft interlayers. • Stress-strain characteristics of soft interlayers are systematically investigated. • Ring shear test is utilized successfully for shear behavior studying. • Variation in compaction moisture content has an obvious effect on shear strength. • Shear behaviors are closely related to the microstructure of soft interlayers. [ABSTRACT FROM AUTHOR]

The salts contained in water can gradually accumulate in pore networks of rocks under wetting-drying cycles, which could cause rock deterioration. The salt-induced damage under this cyclic process can result in dynamically evolving mechanical and hydraulic properties of the rock, which is relevant to many rock-related engineering activities and geological hazards. In this study, the structure damage and permeability evolution of Three Gorges Reservoir (TGR) sandstone subjected to wetting-drying cycles with salt solution were investigated by a series of multiscale experiments. The results showed that the salt solution has a significant impact on the physical properties of TGR sandstone during the wetting-drying process. With increasing wetting-drying cycles, two typical macroscopic damage modes in form of surface peeling and internal cracking were observed. Various physical parameters indicated that the internal structure of the TGR sandstone experienced two stages: 1) Salt accumulation and new crack formation stage and 2) Crack propagation stage. This variation also profoundly affected the permeability of the TGR sandstone. Specifically, the permeability of TGR sandstone decreased slightly at first due to blocked seepage channels by crystals before cycle 5, followed by a slow growth until the 15th cycle and a rapid increase after the 15th cycle. Cubic and exponential functions showed good performances to describe the evolution features of permeability with porosity and wetting-drying cycles, respectively. The deterioration mechanism was further elucidated by an evaporation test. This test indicated that the salt solution was more likely to crystallize in narrow pores and expectedly exert pressure enough to crack the pores, contributing to the structure damage feature and the permeability enhancement. These findings and results in this study deepen our understanding of salt-induced rock deterioration mechanism under the wetting-drying cycles and would be meaningful for rock engineering safety and geological disaster forecast in complex groundwater environments. • Structure damage of TGR sandstone by salt solution under wetting-drying cycles is systematically revealed. • Permeability enhancement of TGR sandstone is functionalized by porosity and wetting-drying cycle. • Salt-induced deterioration mechanism is elucidated by its crystallization features of location and pressure. [ABSTRACT FROM AUTHOR]

The construction of the huge Three Gorges reservoir affected a large region, and the resultant geological and environmental impacts have caused global concern. The remarkable, 30-m annual fluctuation in the reservoir water level poses a significant threat to slope stability in this area. Four hundred sixty-two landslides were identified in the Zigui basin using historical records, satellite images, field investigations and unmanned aerial vehicle (UAV) observations, enabling the construction of a complete landslide database and distribution map. Three failure modes of landslides in the Zigui basin are used to illustrate the major factors that govern reservoir-induced landslides. The results show that (1) > 99% of identified landslides occur on slopes angle <47°, while >80% occur at elevations below 600 m; (2) Jurassic Niejiashan Formation is highly prone to landslides; (3) Low reservoir levels of 145 m to 155 m greatly reduce slope stability. Based on the information entropy method that use the conditional probability of different influencing factors and principal component analysis as inputs, the relative contributions of various influencing factors are quantified and a landslide susceptibility map was drawn. This susceptibility map helps define the countermeasures that will best reduce fatalities and property losses for areas having different landslide conditions and susceptibilities. • Database of 462 reservoir-induced landslides in Zigui basin, TGR was created. • Identifying failure modes and quantifying contributions of influential factors. • Conditional probability and information entropy model yields landslide susceptibility. • Susceptibility map, failure modes, and strategies suppressing the slope instability. [ABSTRACT FROM AUTHOR]